This invention relates to code division multiple access (CDMA) communication between a base station and a mobile station typically through a communication satellite and, more particularly, to a code division multiple access communication method and a base and a mobile station for use in the code division multiple access communication.
A satellite communication network comprises a plurality of terrestrial stations in a service area within which at least one communication satellite is always in line of sight. Each terrestrial station can exchange radio communication to and from a selected one of the other terrestrial stations through at least one of the communication satellites. It is possible for the terrestrial stations to concurrently access at least one of the satellites by multiple access techniques including frequency division, time division or code division multiple access.
Ordinarily, one of the terrestrial stations serves as a source station for a particular communication signal and another, as a destination station for the particular communication signal. The terrestrial stations often comprise at least one base station and a plurality of mobile stations movable in the service area. One of the mobile stations will be taken into consideration as a representative and will he referred to simply as a mobile station.
When transmitted from the base station to the mobile station, the communication signal is herein called a downward transmission signal. The mobile station receives the downward transmission signal as a downward reception signal. Alternatively, the mobile station transmits an upward transmission signal to the base station. The upward transmission signal is received by the base station as an upward reception signal.
According to the code division multiple access technique, each code division multiple access communication network is assigned with a particular code, typically, a particular spectrum spread pattern. In order to provide such a spectrum spread pattern, the base station comprises a base station spectrum spread pattern generator for generating a base generated spectrum spread pattern. The mobile station comprises a mobile station spectrum spread pattern generator for generating a mobile generated spectrum spread pattern. The base station transmits the downward transmission signal by modulating a base transmission signal by the base generated spectrum spread pattern. The mobile station transmits the upward transmission signal by modulating a mobile transmission signal by the mobile generated spectrum spread pattern.
With a propagation delay dependent on a distance between the base and the mobile stations, the downward transmission signal is received as the downward reception signal and the upward transmission signal as the upward reception signal. Consequently, the base station comprises a base station synchronizing unit for synchronizing the base generated spectrum spread pattern with the upward reception signal to produce a base synchronized spectrum spread pattern. The mobile station comprises a mobile station synchronizing unit for synchronizing the mobile generated spectrum spread pattern with the downward reception signal to produce a mobile synchronized spectrum spread pattern. The base station demodulates the upward reception signal into a base reception signal by the base synchronized spectrum spread pattern. The mobile station demodulates the downward reception signal by the mobile synchronized spectrum spread pattern.
In a code division multiple access communication network, each terrestrial station can deal with only a small number of communication. The code division multiple access communication is nevertheless suitible when the terrestrial stations are great in number. In addition, the code division multiple access communication is scarcely adversly affected by interference and disturbances. This applies to communication between a base station and a great number of mobile stations.
It should, however, be noted that the base generated spectrum spread pattern is produced in synchronism with a base station absolute time. The downward transmission signal or signals therefore have frames based on the base station absolute time. The mobile generated spectrum spread pattern is generated in synchronism with a mobile station absolute time which is specific or individual to the mobile station under consideration. The upward transmission signals have frames based on individual mobile station absolute times. For reception as the upward reception signal from one of the mobile stations that is referred to merely as the mobile station in the foregoing, the upward transmission signal must have frames which are not based on the mobile generated or synchronized spectrum spread pattern specific thereto but are related to the base station absolute time. It is, however, difficult due to either a propagation delay or a variation therein to determine a transmisson timing of the upward transmission signal. This has given rise to quality and sensitivity deteriorations of the base reception signal.